Conventional selenium detectors include using an insulating detector top contact and using multi-layer inorganic blocking contact devices. However, neither approach yields photo-gain nor do they use a blocking layer on the bottom of the detector as an anti-crystallization layer. Use of anti-crystallization layers is reported in U.S. Pat. No. 7,649,177 which describes using an inorganic conductive arsenic doped selenium layer.
Conventional avalanche selenium detectors include vacuum tube readout devices and devices using a resistive interface layer based on cellulose. The disadvantage with the former is the need for a high vacuum in order for the pixel to be read out. The disadvantage with the latter is the process complexity associated with multiple blocking layers and lower large area reliability.
In conventional systems, soft polymer layers have been used as top blocking contacts to reduce leakage currents. However, soft polymer layers have, in conventional devices, contributed to charge buildup and lower reliability.
Conventional lateral amorphous-selenium (a-Se) metal-semiconductor-metal (MSM) photodetectors may be used in indirect conversion X-ray imaging applications due to their ease of fabrication and higher speeds when compared to direct conversion a-Se systems. Conventional indirect conversion X-ray imagers couple a visible light-emitting scintillator to an optical detector such as an amorphous-silicon photodiode. Lateral a-Se indirect conversion devices may be individual detectors integrated with a thin-film transistor (TFT) imaging array for use in indirect conversion X-ray imaging. A limitation with conventional lateral detectors is a significant increase in dark current with an increasing applied bias. High biases are desirable to provide a higher photogeneration efficiency and carrier mobility that results from increasing the electric field within the a-Se layer. High dark current leads to a smaller ratio between the photo and dark current at high biases and ultimately limits the minimum detectable light level affecting the dynamic range.
In conventional vertical a-Se X-ray detectors different strategies have been used to mitigate injection of charge into the bulk photodetector. For example, p- and n-like layers are used to block holes and electrons, respectively. However, there may be complexities associated with depositing low-leakage p- or n-like blocking layers. In another conventional design, a single insulating blocking contact near the positively biased electrode is used to block holes only. For detecting visible light photons, vertical a-Se devices may have disadvantages as light must pass through either the top or bottom contact and a blocking layer. As such, there may be a loss of an optical signal compared to lateral photoconductors unless specialized contact layers, like beryllium or transparent conductive oxides, are used.
It is, therefore, desirable to provide faster device or photodetector operation for emerging high speed X-ray imaging applications, higher image quality with amorphous selenium photoconductor imagers, ease of fabrication by using same manufacturing process, and low cost impact.
It is also desirable to provide a photodetector with high reliability, photogain, high responsivity, low dark current, and/or a high speed of operation.